EFF decodes color printer watermarks [LWN.net]

It has been known for some time that high-resolution color printers added codes to their output which would enable that output to be traced. The EFF has now found and decoded those marks for a number of popular printers. It turns out that the scheme used is fairly simple - an unencrypted code which includes the printing time and the serial number of the printer. See the EFF's printer list to see if your printer encodes this information, and this page to learn how to find and decode the markings.

The moral of the story is clear: if we do not control our devices, they will not work in our interests. There are plenty of good reasons for wanting to be able to print anonymously, and there is no doubt that this sort of watermarking can be used for the suppression of dissent and the shutting down of whistle-blowers. Thanks to the EFF, we can at least see this particular bit of technological ratware. But, as the EFF says: "Even worse, it shows how the government and private industry make backroom deals to weaken our privacy by compromising everyday equipment like printers. The logical next question is: what other deals have been or are being made to ensure that our technology rats on us?"

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EFF decodes color printer watermarks

Posted Oct 20, 2005 18:08 UTC (Thu) by jzbiciak (✭ supporter ✭, #5246) [Link]

Now that this is decoded, how long until someone writes a print filter that either forges this, or scrambles it?

EFF decodes color printer watermarks

Posted Oct 23, 2005 2:28 UTC (Sun) by zblaxell (subscriber, #26385) [Link]

Indeed. It seems to me that the error correction is weak, and the printers are probably not smart enough to prevent users from intentionally writing these marks (and what happens if you print a high-resolution random-yellow-dot background?). All you'd have to do is tile the background with multiple overlapping copies of these marks and whoever is looking for you will see hundreds of possible candidates, or they won't be able to separate signal from noise at all. Even better if you're printing this on a printer that doesn't generate the watermarks itself, because whoever's looking for you will look at the document and say "Aha! DocuColor watermarks, this should be easy" prior to embarking on a wild goose chase.

If I had a bit of Postscript code that generated these marks randomly I'd use it as the background to every document I print in the future, along with lots of those little circle patterns designed to prevent people from forging Euros. The latter is really fun for small offices that can't afford unencumbered printers and photocopiers, when they try to make a copy of some form you filled out for their files.

Now, if I were evil and I could interfere with consumer products at will, I'd put microscopic ID tags in the *toner*. One advantage of this approach is that you're likely to find the ID tags on both the pamphlets and the pamphleteer, if said pamphleteer has ever fixed a paper jam, or just rubbed printed paper against their clothing. That would take care of all those pesky EFF activists with their modchipped printers...

All dissent should be black and white then.

Posted Oct 21, 2005 15:13 UTC (Fri) by smoogen (subscriber, #97) [Link]

Ah, but how many black and white printers have the same problem in them? It is cheaper to reuse a chipset between printers and probably code that is neutral.

On the other hand, I dont see anyone making a mass-market 'Free' printer anytime soon.

EFF decodes color printer watermarks

Posted Oct 27, 2005 7:15 UTC (Thu) by landley (guest, #6789) [Link]

A friend of mine mentioned that this explains why people who print stacks
of black and white documents run out of yellow ink, which is not just
annoying but _expensive_.

Personally, I object to the Fatherland Security types intentionally
wasting something I have to pay for. The fact it costs more for per ounce
than gold just adds insult to injury.

Can I send them a bill?

Rob